This invention relates to a method for removing volatile components such as water and volatile organic compounds such as solvents from polymer powders using an extruder. The method of the present invention avoids excessive backflow of the volatile components through the extruder feed throat, a condition which severely limits throughput rates.
The manufacture of various polymers frequently involves a final drying step in order to remove water and other volatile components utilized in the process used to prepare the polymer. In some instances, the drying process involves treating a wet polymer powder containing volatile components such as solvents or other volatile materials in a fluidized bed post dryer. Such equipment is frequently large and costly, comprising large industrial dryers, nitrogen delivery systems, and scrubber systems to control the unwanted escape of volatile components into the environment. While such fluidized bed dryers are typically effective at reducing the moisture and volatile organic content of a variety of polymer powders, processes using them are relatively slow and costly.
During the drying of polymer powders, rates of drying may be dependent upon the particle size and particle size distribution. For example, typical polycarbonate powders isolated following the interfacial polymerization of a dihydroxy aromatic compound with phosgene have a broad distribution of particle sizes ranging from less than 100 micrometers to well above 1 millimeter. The number-averaged particle size is typically less than 200 micrometers. When the moisture level present in a polymer powder falls below a critical moisture content, drying rates may be limited by the rates of liquid diffusion and capillary transport in the powder particles. Thus, the last stages in the drying of a polymer powder may be difficult and require long drying times. It would be desirable, therefore, to discover means for the efficient removal of volatile components from polymer powders which does not involve the use of complex equipment such as fluidized bed post-dryers. In particular, it would be desirable to discover a highly efficient means of removing volatile components from wet polycarbonate powders.
The present invention provides an alternate method for drying solid polymer powders containing volatile components such as water and organic solvents using a devolatilization extruder. The method employs standard equipment of a type readily available in typical polymer finishing facilities and may be adapted to include other operations involving the polymer as well, such as blending with other polymers, functionalization by chemical reaction, and controlled molecular weight adjustment. The present invention reduces the need for fluidized bed-type drying operations, and allows the conversion of solid polymer powders containing high levels of water into polymer compositions which are substantially free of water in a single extrusion step. The removal of water from the polymer powder using the method of the present invention also produces as an unexpected benefit, the simultaneous removal of process solvents and trace impurities, such as residual monomer and low molecular weight oligomers (xe2x80x9clowsxe2x80x9d), which can impact product characteristics such as plate-out during molding of the final polymer composition. The method of the present invention represents a simple, low cost advance in polymer drying technology, and may afford polymer compositions having improved product properties relative to polymer compositions prepared using conventional drying technology.
In contrast to the present invention, in which a polymer powder containing volatile components is subjected to extrusion devolatilization, the extrusion devolatilization of polymers in solution is well known. For example, Curry and Brauer in Polymer Devolatilization, R. Albalak Ed. Marcel Deker Inc., p 345, 1996, describe the conversion of a solution of high density polyethylene in cyclohexane into a solid polymeric material containing about 10% by weight cyclohexane, using an extruder based process comprising rapid removal of solvent from a superheated solution of the polymer in cyclohexane in an extruder, said extruder being equipped for multistage stripping agent injection and venting.
The process described by Curry and Brauer, however, is inapplicable to the removal of substantial quantities of volatile substances from polymeric materials in powder form, such as wet polycarbonate powders isolated following interfacial polymerization. Such polycarbonate powders typically contain between about 1 and about 20 percent by weight water, and between about 0.001 and about 5 percent by weight methylene chloride. Attempts to devolatilize such powders using an extruder are hampered by the tendency of the steam generated as the polycarbonate powder is sheared and heated above its glass transition temperature to xe2x80x9cback outxe2x80x9d of the feed throat of the extruder. The steam fluidizes the powdered polymeric material being introduced at the feed throat and causes feeding problems, such as powder escaping from the feed throat and powder xe2x80x9cbridgingxe2x80x9d at the feed throat. Feeding problems of this type may be eased somewhat through the use of auxiliary devices, such as screw feeder stuffers and crammer feeders. However, such devices are frequently ineffective since the fine powders propelled by steam escaping from the extruder may still escape from the stuffers and crammer feeders. In addition such auxiliary devices further increase the cost and complexity of the operation.
Another potential solution to the feeding problems described above is the employment of additional step to compact or agglomerate the wet powder into larger particles prior to introduction of the powder into the extruder. Again, this requires dedicated equipment for additional process steps, resulting in increased process complexity, higher capital and operating costs.
U.S. Pat. No. 5,232,649 discloses a process which uses the mechanical force of the screws in a twin screw extruder to squeeze water and other volatiles from a wet polymeric material. The water is removed from the polymer in a liquid state rather than as steam. While such a process works well for polymeric materials having low glass transition temperatures (Tg), for example materials having Tg""s below 100xc2x0 C., it is inapplicable to the efficient removal of water and other volatile substances from higher Tg polymeric materials such as polycarbonate due to severe limitations of throughput rates.
U.S. Pat. No. 5,717,055 discloses a process for producing polycarbonate pellets by melt-kneading a polycarbonate power in the kneading section of an extruder which comprises a xe2x80x9cmelt sealxe2x80x9d mechanism consisting of reverse-flighted screw elements or a dam ring. High vacuum is applied at a vent downstream of the xe2x80x9cmelt sealxe2x80x9d to remove volatile components. It is well known, however, that when reverse-flighted (left handed) screw elements are used, the upstream screw section must be completely filled over a certain distance in order to generate the pressure necessary to override the reverse-flighted (left handed) section (Rauwendaal, Plastics Formulating and Compounding, Nov./Dec., 1995). Thus, steam generated in the screw sections preceeding the reverse-flighted section is forced to flow back along the screws and out of the feed throat of the extruder. Again, the emerging steam fluidizes the powdered polymeric material being fed to the extruder and may cause feeding problems. Where the polymer being introduced into the extruder contains a substantial amount of water and other volatile components, the rate of introduction of the polymer must be limited in order to avoid the effects of large amounts of steam escaping through the feed throat and only a fraction of the total torque and power available for polymer processing is used. For productivity reasons, it is a disadvantage not to be able to operate the extruder near the torque and power limitation of the extruder, as in the case of dry powder compounding.
U.S. Pat. No. 4,845,193 discloses a process for producing a low particle polycarbonate comprising supplying a wet polycarbonate powder having a water content of from 0.5 to 10 percent by weight and an organic solvent content of from 0.01 to 5 percent by weight to a vented extruder. The reference discloses a conventional screw design and product output rates fall well below those achieved using the method of the present invention.
Japanese Patent, JP09193230, discloses a process of injecting water into molten polycarbonate in the kneading portion of an extruder as a means of removing volatile components of the polycarbonate. In this case, however, the polymer is already molten at the point at which the water is introduced and steam is not forced back along the screws to the feed throat. The process is inapplicable in cases where the polymeric material to be extruded is a powder containing a substantial amount of water such as wet polycarbonate powder containing methylene chloride.
European Patent Application 0410650 A1 describes a method for preparing polyamide prepolymers by introducing the constituent monomers into an extruder comprising a powder seal. The reference does not disclose the method of the present invention.
The present invention provides a method for removing volatile components from a polymer powder, said method comprising:
Step (A) introducing a polymer powder comprising water into an extruder, said extruder comprising;
(i) a powder conveying section;
(ii) a powder seal section located downstream of said powder conveying section;
(iii) a kneading and melting section located downstream of said powder seal section; and
(iv) a vacuum vent located downstream of said kneading and melting section;
Step (B) conveying the polymer powder through said powder seal section;
Step (C) heating and shearing the polymer powder in said kneading and melting section to form a polymer melt comprising water; and
Step (D) subjecting said polymer melt to vacuum venting at said vacuum vent.
The present invention further relates to a method for performing a second operation involving the polymer powder, such as blending, compounding or chemical modification, during extruder devolatilization. In one aspect of the present invention, this second operation comprises the introduction of a hydrolysis catalyst into the extruder during the extruder devolatization of wet polycarbonate powder and effecting a controlled molecular weight reduction of the polycarbonate. In another aspect of the present invention a polymer blend is prepared during the extruder devolatilization.